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mm, slub: avoid zeroing kmalloc redzone
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Since commit 946fa0d ("mm/slub: extend redzone check to extra
allocated kmalloc space than requested"), setting orig_size treats
the wasted space (object_size - orig_size) as a redzone. However with
init_on_free=1 we clear the full object->size, including the redzone.

Additionally we clear the object metadata, including the stored orig_size,
making it zero, which makes check_object() treat the whole object as a
redzone.

These issues lead to the following BUG report with "slub_debug=FUZ
init_on_free=1":

[    0.000000] =============================================================================
[    0.000000] BUG kmalloc-8 (Not tainted): kmalloc Redzone overwritten
[    0.000000] -----------------------------------------------------------------------------
[    0.000000]
[    0.000000] 0xffff000010032858-0xffff00001003285f @offset=2136. First byte 0x0 instead of 0xcc
[    0.000000] FIX kmalloc-8: Restoring kmalloc Redzone 0xffff000010032858-0xffff00001003285f=0xcc
[    0.000000] Slab 0xfffffdffc0400c80 objects=36 used=23 fp=0xffff000010032a18 flags=0x3fffe0000000200(workingset|node=0|zone=0|lastcpupid=0x1ffff)
[    0.000000] Object 0xffff000010032858 @offset=2136 fp=0xffff0000100328c8
[    0.000000]
[    0.000000] Redzone  ffff000010032850: cc cc cc cc cc cc cc cc                          ........
[    0.000000] Object   ffff000010032858: cc cc cc cc cc cc cc cc                          ........
[    0.000000] Redzone  ffff000010032860: cc cc cc cc cc cc cc cc                          ........
[    0.000000] Padding  ffff0000100328b4: 00 00 00 00 00 00 00 00 00 00 00 00              ............
[    0.000000] CPU: 0 UID: 0 PID: 0 Comm: swapper/0 Not tainted 6.11.0-rc3-next-20240814-00004-g61844c55c3f4 torvalds#144
[    0.000000] Hardware name: NXP i.MX95 19X19 board (DT)
[    0.000000] Call trace:
[    0.000000]  dump_backtrace+0x90/0xe8
[    0.000000]  show_stack+0x18/0x24
[    0.000000]  dump_stack_lvl+0x74/0x8c
[    0.000000]  dump_stack+0x18/0x24
[    0.000000]  print_trailer+0x150/0x218
[    0.000000]  check_object+0xe4/0x454
[    0.000000]  free_to_partial_list+0x2f8/0x5ec

To address the issue, use orig_size to clear the used area. And restore
the value of orig_size after clear the remaining area.

When CONFIG_SLUB_DEBUG not defined, (get_orig_size()' directly returns
s->object_size. So when using memset to init the area, the size can simply
be orig_size, as orig_size returns object_size when CONFIG_SLUB_DEBUG not
enabled. And orig_size can never be bigger than object_size.

Fixes: 946fa0d ("mm/slub: extend redzone check to extra allocated kmalloc space than requested")
Cc: <[email protected]>
Reviewed-by: Feng Tang <[email protected]>
Acked-by: David Rientjes <[email protected]>
Signed-off-by: Peng Fan <[email protected]>
Signed-off-by: Vlastimil Babka <[email protected]>
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@MrVan @tehcaster
MrVan authored and tehcaster committed Sep 3, 2024
1 parent 3beb2fb commit 59090e4
Showing 1 changed file with 53 additions and 47 deletions.
100 changes: 53 additions & 47 deletions mm/slub.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -756,6 +756,50 @@ static inline bool slab_update_freelist(struct kmem_cache *s, struct slab *slab,
return false;
}

/*
* kmalloc caches has fixed sizes (mostly power of 2), and kmalloc() API
* family will round up the real request size to these fixed ones, so
* there could be an extra area than what is requested. Save the original
* request size in the meta data area, for better debug and sanity check.
*/
static inline void set_orig_size(struct kmem_cache *s,
void *object, unsigned int orig_size)
{
void *p = kasan_reset_tag(object);
unsigned int kasan_meta_size;

if (!slub_debug_orig_size(s))
return;

/*
* KASAN can save its free meta data inside of the object at offset 0.
* If this meta data size is larger than 'orig_size', it will overlap
* the data redzone in [orig_size+1, object_size]. Thus, we adjust
* 'orig_size' to be as at least as big as KASAN's meta data.
*/
kasan_meta_size = kasan_metadata_size(s, true);
if (kasan_meta_size > orig_size)
orig_size = kasan_meta_size;

p += get_info_end(s);
p += sizeof(struct track) * 2;

*(unsigned int *)p = orig_size;
}

static inline unsigned int get_orig_size(struct kmem_cache *s, void *object)
{
void *p = kasan_reset_tag(object);

if (!slub_debug_orig_size(s))
return s->object_size;

p += get_info_end(s);
p += sizeof(struct track) * 2;

return *(unsigned int *)p;
}

#ifdef CONFIG_SLUB_DEBUG
static unsigned long object_map[BITS_TO_LONGS(MAX_OBJS_PER_PAGE)];
static DEFINE_SPINLOCK(object_map_lock);
Expand Down Expand Up @@ -985,50 +1029,6 @@ static void print_slab_info(const struct slab *slab)
&slab->__page_flags);
}

/*
* kmalloc caches has fixed sizes (mostly power of 2), and kmalloc() API
* family will round up the real request size to these fixed ones, so
* there could be an extra area than what is requested. Save the original
* request size in the meta data area, for better debug and sanity check.
*/
static inline void set_orig_size(struct kmem_cache *s,
void *object, unsigned int orig_size)
{
void *p = kasan_reset_tag(object);
unsigned int kasan_meta_size;

if (!slub_debug_orig_size(s))
return;

/*
* KASAN can save its free meta data inside of the object at offset 0.
* If this meta data size is larger than 'orig_size', it will overlap
* the data redzone in [orig_size+1, object_size]. Thus, we adjust
* 'orig_size' to be as at least as big as KASAN's meta data.
*/
kasan_meta_size = kasan_metadata_size(s, true);
if (kasan_meta_size > orig_size)
orig_size = kasan_meta_size;

p += get_info_end(s);
p += sizeof(struct track) * 2;

*(unsigned int *)p = orig_size;
}

static inline unsigned int get_orig_size(struct kmem_cache *s, void *object)
{
void *p = kasan_reset_tag(object);

if (!slub_debug_orig_size(s))
return s->object_size;

p += get_info_end(s);
p += sizeof(struct track) * 2;

return *(unsigned int *)p;
}

void skip_orig_size_check(struct kmem_cache *s, const void *object)
{
set_orig_size(s, (void *)object, s->object_size);
Expand Down Expand Up @@ -1894,7 +1894,6 @@ static inline void inc_slabs_node(struct kmem_cache *s, int node,
int objects) {}
static inline void dec_slabs_node(struct kmem_cache *s, int node,
int objects) {}

#ifndef CONFIG_SLUB_TINY
static bool freelist_corrupted(struct kmem_cache *s, struct slab *slab,
void **freelist, void *nextfree)
Expand Down Expand Up @@ -2239,14 +2238,21 @@ bool slab_free_hook(struct kmem_cache *s, void *x, bool init)
*/
if (unlikely(init)) {
int rsize;
unsigned int inuse;
unsigned int inuse, orig_size;

inuse = get_info_end(s);
orig_size = get_orig_size(s, x);
if (!kasan_has_integrated_init())
memset(kasan_reset_tag(x), 0, s->object_size);
memset(kasan_reset_tag(x), 0, orig_size);
rsize = (s->flags & SLAB_RED_ZONE) ? s->red_left_pad : 0;
memset((char *)kasan_reset_tag(x) + inuse, 0,
s->size - inuse - rsize);
/*
* Restore orig_size, otherwize kmalloc redzone overwritten
* would be reported
*/
set_orig_size(s, x, orig_size);

}
/* KASAN might put x into memory quarantine, delaying its reuse. */
return !kasan_slab_free(s, x, init);
Expand Down

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